Architecture decision records

Short documents recording decisions that were not obvious, and why the alternative was rejected. Kept because the reasoning is otherwise lost, and someone reasonably proposes the alternative again a year later.

Two records are referenced from the source (wrappers.py cites ADR-0003, validation.py cites ADR-0006) but were never written. The decisions they describe are documented in the pages below until they are:

Decision	Where it is written up
Property wrappers as unchecked promises	Structure and laziness
Validation as an explicit call rather than a constructor check	Debugging
sqrt returns a left square root	Decompositions
Symmetry enforced per operation, not at construction	Structured operators
Both CG modes rather than choosing one	Solving

Format

not-executed

# ADR-NNNN: Title

## Status
Accepted | Superseded by ADR-MMMM

## Context
What forced a decision.

## Decision
What was decided.

## Consequences
What this makes easy, and what it makes hard.